The objective of gas to gas heat and moisture transfer devices is to transfer heat from a hot gas stream to a cold gas stream and or to transfer moisture from a moist gas stream to a relatively dry gas stream to reduce the consumption of energy in an industrial process or in heating and air conditioning of a building. In practice the moisture is so frequently water and the gas so frequently air that this combination will provide the basis for the description of this invention. It is important to emphasize, however, that the equipment, techniques, and relationships are equally applicable to other systems as well.
There are three general groups of applications where air to air heat and moisture transfer devices are being used, including:
process-to-process applications, where heat and moisture is transferred from a hot and or moist waste gas stream into incoming cool or dry process gas stream, PA1 process-to-comfort applications, where heat is transferred from a hot waste gas stream into an air stream used to heat or cool a building, and PA1 in comfort-to-comfort applications, where heat and moisture is transferred between the exhaust air and the supply air streams, with the enthalpy of the supply air being lowered during summer and raised during the winter. PA1 capability to function as sensible heat transfer devices, moisture transfer devices, and total energy transfer devices, PA1 a much more compact heat and or moisture transfer surface can be used, PA1 the heat and moisture transfer surface in general is substantially less expensive, PA1 the periodic flow reversals eliminate permanent flow stagnation regions negatively affecting performance and effectiveness, PA1 due to flow reversals the heat and or moisture transfer surfaces tend to be self cleaning, PA1 the relatively simple modulation of the operating cycle time permits inexpensive modulation of the performance and effectiveness.
Devices capable of transferring of only the sensible heat are known as the sensible heat exchangers, those capable of transferring heat and moisture as total energy exchangers and or as adsorption or absorption dehumidifiers.
The two types total energy exchangers and or dehumidifiers capable of transferring heat and moisture between two air streams are the rotary wheel types and the valved types exchangers. Both types have the capability and are also used as only the sensible heat exchangers.
In both moisture as well as heat is transferred between the two air streams by first absorbing heat and or moisture from the first hot and or moist air stream into a porous matrix of a suitable solid material during the first, i.e. sorption period, and then by releasing the heat and or moisture from the matrix into the second a relatively cool and or dry air stream during the second, i.e. desorption period. The two sorption-desorption periods represent the operating cycle of the two types matrix based heat and moisture transfer devices.
In the rotary types, the rotation of the matrix provides the flow of the solid phase from the hot and or moist side to the cool and or dry side air streams in a regular periodic manner, with the matrix being alternately saturated with moisture and then dried, and or heated and then cooled, with the moisture and or heat being transferred indirectly from the moist and or hot air stream to the relatively dry and or cool air stream.
In the valved types, using one or more identical matrices, each matrix is functioning alternately as either the moist and or hot flow or a dry and or cold flow matrix, with the two air streams being regularly switched between the two matrices by means of a periodic switching of quick operating valves.
The heat and mass transfer theory which applies equally to both types has been described elsewhere (W.M. Kays, A. L. London, "Compact heat exchanges", McGraw Hill Book Co., sec.ed.,1964; W. H. McAddams, "Heat Transmission", 3rd ed., McGraw Hill Book Co., 1954; P. C. Wancat, "Large-scale Adsorption and Chromatography", CRC Press Inc., 1986).
The major advantages of the periodic flow type heat and moisture exchangers compared with the direct type heat exchangers are:
A recognized disadvantage of the matrix based devices is some mixing of the two air streams due to carry over and leakage.
Since the two air streams are usually at different pressures, and since perfect sealing of large rotary surfaces is difficult if not impossible, the leakage of the rotary types is practically unavoidable. With the valved types the leakage can be eliminated.
The carry over for both types depends on the operating cycle time and on the void of the matrix, and for both types can be reduced or eliminated by purging of the matrix for the rotary types, or by discarding the contaminated portion of the clean air stream for the valved types.
A disadvantage of the rotary types is a relatively expensive matrix, housing, seals and drives, and a disadvantage of the valved types is the relatively expensive valving, piping or ducting associated with considerable flow resistances affecting consumption of electric power in used air movers.
Consequently, the objective of the present invention is to provide an improved periodic flow type heat exchanger-dehumidifier with reduced mixing of the two air streams, with reduced flow resistances, with easy modulation of the performance and effectiveness and with reduced capital and operating costs for use in the various proces-to-process, process-to-comfort, and comfort-to-comfort gas to gas heat and or moisture transfer applications.